Fuel cells convert chemical energy from a fuel into electricity, by using an electrochemical process such as a chemical reaction of positively charged hydrogen ions with oxygen or another oxidizing agent. Fuel cell operation generally depends on two supplies: a fuel supply which provides an ongoing source of fuel, and an oxygen supply which provides an ongoing source of oxygen to sustain the chemical reaction. A variety of fuels are in use or under consideration for use in fuel cells. Various oxidizers are also used or under consideration; in some cases, ambient air serves as an oxygen supply. Unlike a battery, which eventually drains and must be recharged before further use as a power source, a fuel cell can produce electricity continuously as long as the necessary fuel and oxygen are supplied.
Fuel cells are used or being considered for use in a wide variety of circumstances, alone or in combination with batteries or internal combustion engines, including for example: commercial, industrial, leisure, manufacturing, medical, military, residential, and scientific primary and backup power generation; as power sources in remote, isolated, or off-grid locations such as cabins, communication centers, communication towers, gas or oil well sites, military facilities, parks, prisons, research stations, rural locations, spacecraft, security posts, and weather stations; and in vehicles such as airplanes, all-terrain vehicles, boats, buses, cars, forklifts, recreational vehicles, spacecraft, submarines, trains, trucks, and unmanned remotely guided or autonomous vehicles.
Fuel cells have various properties that can help promote efficiency, reliability and adaptability as a power source. For example, a fuel cell can be compact and lightweight, and many fuel cells do not require any moving parts to operate. Fuel cell power generation can be environmentally much cleaner than fossil fuel power generation. Some fuel cells can use low-quality gas from landfills or waste-water treatment plants to generate power and simultaneously lower methane emissions. The theoretical energy efficiency, and in many cases the actual real-world energy efficiency, of a fuel cell can be substantially greater than the energy efficiency of fossil fuel power generation or other power sources. Fuel cells are also versatile. By consuming the oxygen within an enclosed space, a fuel cell can be used for food preservation. In some breathalyzers, the amount of voltage generated by a fuel cell is used to determine the concentration of fuel (alcohol) present in a sample. There are many other examples as well.
However, like any power source, the particular characteristics of fuel cells can be advantages or challenges in a given situation. Ongoing consumption of oxygen, for example, is an advantage when removal of oxygen from a shipping container helps preserve food that is stored in the container, but oxygen consumption is a significant challenge to the use of a fuel cell in limited-oxygen environments such as space stations and spacecraft. Accordingly, close and thoughtful attention to the characteristics of fuel cells in a given situation may reveal technical problems.